The present invention relates generally to nuclear fuel assembly, and more particularly to a highly portable device and related method for controlling the reactivity of a fuel assembly outside of the nuclear reactor vessel.
Most high capacity dry storage canisters used to store commercial nuclear fuel, such as MPC-32 in the HI-STORM 100 system (USNRC Docket #72-1014) are of the so-called Non-flux trap (NFT) type which means a single panel of neutron absorber lies between two facing fuel assemblies. The storage cells are thus tightly packed and the number of fuel assembles that can be accommodated in a cask (or canister) of a given cross section is maximized. These high capacity fuel baskets, however, suffer from the demerit that they do not meet the NRC's (Nuclear Regulatory Commission's) sub-criticality criterion (reactivity multiplier <0.95 with all uncertainties and biases factored in with 95% confidence and 95% certainty) for fresh fuel of a relatively high initial enrichment (say, 5 w/0 U-235). Even the most modern PWR fuel basket, such as the one in MPC-37 (NRC Docket #72-1032) cannot satisfy the above criticality requirement for the largest commercial PWR fuel assemblies in spite of the fact that it features neutron absorber panels with substantially greater boron areal concentration that older basket designs. Areal density of boron is the direct indicator of the neutron absorption capacity of a neutron absorber.
This deficit in the neutron absorption ability of the high capacity fuel basket is overcome by relying on the soluble boron in the fuel pool's water (permitted under USNRC's 10 CFR 72 rules) while the cask/canister is being loaded in the fuel pool. During transport, the U.S. NRC allows partial credit for fuel burn-up (USNRC ISG-8) under 10 CFR 71 rules thus enabling the high capacity fuel baskets to be used to both store and transport spent nuclear fuel.
This condition of reliance on regulatory dispensation is, however, not entirely satisfactory, because the extent of burn-up credit allowed by different regulatory jurisdictions varies widely and the actual amount of burn-up exposure garnered by a fuel assembly is subject to some uncertainty. Evidently, it would be far better to equip the fuel with additional neutron absorption capability such that no reliance on boron credit or burn-up credit is necessary.
An improved approach is desired for storing nuclear fuel and controlling reactivity.